Magnetoresistors are devices whose resistance varies with a magnetic field applied to the device and are therefore useful for magnetic field sensors. Hall effect devices and a gateless version of the split drain magnetotransistor are related magnetic field sensors which can be made with the same fabrication technology. The magnetoresistor is useful for position sensing applications and may be useful for a variety of other applications, such as brushless motors or magnetic memory storage readout devices. Initially, magnetoresistors were believed to be best formed from high carrier mobility semiconductor materials in order to obtain the highest magnetic sensitivity. Therefore, the focus was on making magnetoresistors from bulk materials that were thinned down or on films having sufficient thickness to exhibit a high average mobility. However, as described in the copending U.S. patent application of D. L. Partin et al., Ser. No. 07/426,245, filed Oct. 25, 1989 entitled, "Magnetoresistor," and assigned to the same assignee as the present application, good magnetoresistors can be made of an extremely thin film of a semiconductor material having an accumulation layer induced in the surface of the film. Semiconductor materials which have been found satisfactory for making such magnetoresistors include indium antimonide. Such magnetoresistors comprise a thin film of the semiconductor material on the surface of a substrate of an insulating material, preferably a semiconductor material having a lattice constant close to that of the semiconductor material of the thin film so as to be able to obtain a film of good crystalline quality. The film is defined to be in the form of a relatively narrow strip having a length substantially greater than its width. Conductive contacts are provided on the film at the ends thereof, and shorting bars of a conductive material are on the film spaced apart along the length of the strip to divide the strip into a plurality of active regions of the appropriate size.
It is important to obtain high crystalline quality of the semiconductor material film in order to have high electron mobility, which produces high sensitivity to magnetic fields. It has been found that another important aspect of the magnetoresistor is the specific contact resistance (r.sub.c) between the semiconductor material film and the contacts and shorting bars. The actual resistance of such a contact (R.sub.c) is determined by dividing the specific contact resistance (r.sub.c) by the area of the contact or shorting bar. The conductive shorting bars can be thought of as periodically shorting out the Hall electric field of the semiconductor film. To do this, the conductive shorting bar must have a low sheet resistance (R.sub.s) relative to the semiconductor material film, and must have a low resistance. The importance of having very low sheet resistance and actual resistance can be appreciated from the fact that there are generally many such contacts in series along a strip of the semiconductor material film, and such (electrical contact) resistances are insensitive to magnetic field.
Although good (low resistance) contacts have been made easily to a number of the semiconductor materials which have been used for magnetoresistors, such as gallium arsenide (GaAs) and indium arsenide (InAs), it has been found that making good contacts to indium antimonide (InSb) is a problem. This was unexpected since, other things being equal, it is generally true that the smaller the energy band gap (Eg) is of a semiconductor material, the smaller the energy barrier is at a contact to it and, therefore, the lower the specific resistance, r.sub.c. Contacts to n-type conductivity gallium arsenide (Eg=1.4 eV) have been previously made using multi-layers of Au-Ge or In-Sn followed by a high temperature alloying step. Contacts to indium arsenide (Eg=0.36 eV) with negligible resistance were easily obtained using gold, tin or indium, with no high temperature annealing required. Since indium antimonide has a very small energy band gap, 0.18 eV, it was believed that there would be no problem in forming a low resistance contact to it. However, we have found that Au and Au-Ge metallization systems are inadequate for the highest performance InSb magnetoresistor. Indium based alloys, such as In-Sn, generally have melting points which are too low for some practical device environments, such as certain automotive applications which require operating temperatures up to 200.degree. C. or higher. Therefore, it is desirable to have a contact for a thin film indium antimonide magnetoresistor which has low sheet resistance, low contact resistance, will withstand the high temperatures at which the magnetoresistor may be used and which can be easily made.